1. Field of the Invention
The present invention relates to the preparation of alkyl adipates from alkyl pentenoates, and, more especially, to the selective preparation of alkyl adipates by reacting carbon monoxide and an alcohol with an alkyl pentenoate.
2. Description of the Prior Art
It is well known to this art [compare Bulletin of the Chemical Society of Japan, 46, pp. 526-527 (1973)] that a mixture containing dialkyl esters, and in particular an alkyl adipate, is obtained by reacting carbon monoxide and an alcohol with an alkyl pent-3-enoate, under high pressure and at elevated temperature, in the presence of cobalt carbonyl and an aromatic heterocyclic nitrogen base. However, the industrial-scale development of a technique of this type, the value of which is not contested in principle, is greatly jeopardized not only by the low efficacy of the catalyst system, but also by the substantial proportion of alkyl pentanoate formed, even though the reaction is carried out in the absence of hydrogen.
Furthermore, it too is well known to this art that the presence of small amounts of hydrogen in the reaction medium tends to increase the efficacy of cobalt-based catalysts in processes for the synthesis of esters by reacting an alcohol and carbon monoxide with an olefinic compound.
It has nevertheless also been found that, in the process in question, this favorable effect associated with the presence of small amounts of hydrogen is accompanied by an adverse influence on the selectivity of the process in respect of alkyl adipates, which are the specifically desired products.
In fact, it has been observed that the presence of hydrogen not only tends to increase the proportion of hydrogenation products in the reaction mixture, but is also capable of reducing the proportion of adipate in the diesters formed.
This adverse effect greatly detracts from the economics of the subject process, insofar as the utilization of the branched diesters and the alkyl pentanoates is uncertain or even nonexistent. In other words, the formation of these products, which are destroyed in practice, corresponds to an intolerable loss of starting material. Furthermore, hydrogen can be formed in situ from the traces of water which may be present in technical-grade reactants, according to the well known reaction: EQU H.sub.2 O+CO.fwdarw.CO.sub.2 +H.sub.2
It would be desirable, for obvious economic reasons, to be able to employ technical-grade carbon monoxide containing hydrogen, without this detracting from the selectivity of the process in respect of alkyl adipates, which are the desired diesters. It would also be desirable, for the same reasons, to be able to use reactants containing traces of water, without this resulting in a loss of starting material.